Dry skin is caused by an inadequate moisture content in the stratum corneum. The stratum corneum is a multicellular membrane of flattened, metabolically active cells which forms the outer layer of skin. The membrane is dynamic, constantly renewing itself as surface cells are lost through desquamation but replaced at an equivalent rate from underlying epidermal cells. This process maintains an essentially constant number of cell and a constant thickness in the stratum corneum.
The stratum corneum's water content must remain not less than approximately 10% to maintain normal skin hydration. At this moisture level, keratin (the horny skin layer) softens and attains a plastic state. This moisture level occurs in normal skin when the environment is at about 60% relative humidity. In the normal indoor environment, moisture content of the stratum corneum is about 10 to 15%. At 95% relative humidity, the stratum corneum's moisture content increases to about 65%. At low temperature and relative humidity, the outer skin layer dries out, becomes less flexible and may crack when flexed thereby increasing the rate of moisture loss.
Dry skin is characterized by one or more of the following: roughness or flaking; loss of flexibility; fissures; hyperkeratosis; inflammation and pruritus. While dry skin can occur at any season, it is especially prevalent in the winter and commonly found on the forearms, back of the hands, fingers and lower legs. Other causes of dry skin include disease, prolonged detergent use, malnutrition, age and physical damage to the stratum corneum.
Water is the only true plasticizer for human stratum corneum. The optimum treatment for dry skin is to raise the stratum corneum's moisture level and to reestablish its integrity. Approaches to treating dry skin include: lubricating the skin; moisturizing the skin; chemically softening the keratinous epidermal layer; treatment with anti-inflammatory medicinal compounds. A detailed discussion of the approaches for treating dry skin is contained in Handbook of Nonprescription Drugs, eighth edition, Copyright 1986, American Pharmaceutical Assoc., Washington, D.C., Chapter 30, pages 597 to 631, the entire contents of which are hereby incorporated by reference.
Moisture diffuses to the keratin layer about 50 to 100 times faster than it is lost from the skin surface. Human skin is an effective barrier against water loss. Physical damage increases transepidermal water loss.
One of the primary treatments of dry skin involves the use of occlusive agents. Occlusives are hydrophobic substances that promote water retention by forming a barrier on the skin that will prevent moisture loss. The most commonly used occlusive agents include petrolatum, lanolin, cocoa butter, mineral oil and silicones.
Occlusives alone are not considered sufficient treatment. Patients are generally directed to soak the effected area in water for 5 to 10 minutes and then immediately apply the occlusive agent. This treatment will hydrate and then trap moisture in the skin. It is also believed that occlusives reestablish the integrity of the stratum corneum. In addition, occlusion may increase the metabolic rate of the epidermis, thereby increasing production of materials that become part of the stratum corneum. Caution must be exercised to avoid excessive hydration and maceration.
The best occlusive agents are, by their very nature, oleaginous having a greasy texture and are difficult to spread. More esthetic oil-in-water emulsions are preferred modes for applying occlusive agents. They are less effective, however, and rely on the aid of other formulating agents to form a film on the skin after the product's water content has evaporated.
While much effort has been directed to providing a highly effective, esthetically pleasing product none have been completely successful. The traditional approach has been to apply the occlusive product and produce the coating film in one step. The net result is that good esthetics are achieved at the expense of good occlusive films. Current products and methods of use have not been able to provide both a highly occlusive film and good esthetics in one product and method of use.
The composition and method of the present invention are directed to adding moisture to dry skin and applying a thin, long lasting occlusive film that is both effective and esthetically pleasing. The essential property of the present skin treating composition is that it increases stratum corneum flexibility by adding and sealing in moisture with a long lasting esthetically pleasing occlusive film.